1. Field of the Invention
The present invention relates to an actuator comprising a guide-equipped frame having guide grooves formed on the frame. The present invention also relates to a method for producing such a guide-equipped frame.
2. Description of the Related Art
Various actuators are conventionally used to transport or position a workpiece. Japanese Laid-Open Utility Model Publication No. 2-12554, for example, discloses an actuator having a guide-integrated frame which has guide grooves integrally formed on inner wall surfaces.
The actuator comprises the guide-integrated frame having ball-rolling grooves (guide grooves) axially extending on the inner wall surfaces on both opposed sides. The guide-integrated frame has a ball screw shaft which extends substantially in parallel to the ball-rolling grooves. Further, the guide-integrated frame has a slider. The slider reciprocates along the ball-rolling grooves under a screwing action by means of the ball screw shaft.
A method for producing the conventional guide-intergrated frame will be briefly explained. A pillar-shaped member is drawn to form a drawn product. Warpage of the drawn product is straightened. Next, cutting machining is performed on outer surfaces of the drawn product that cannot be straightened. Then straightening is performed again.
Next, a hardening process, such as vacuum hardening or high frequency hardening, is performed. Thereafter, straightening and polishing are performed on the outer surface. Groove-polishing is also performed to form ball-rolling grooves along the inner wall surfaces, using a disk-shaped grinding wheel or the like. Thus, the guide-integrated frame is completed.
However, a large number of treatment steps are required in the method for producing the conventional guide-integrated frame. Therefore, production costs are high. Further, it is impossible to improve production efficiency, because an extremely long period of time is required to polish the outer surface.
U.S. Pat. No. 5,755,515 discloses an actuator body having a base portion fabricated from a light metal or a light metal alloy and supporting a slider, made of the same light metal or light metal alloy as the base, for reciprocal movement along the base portion. The slider and base portions of the actuator include respective guide rails. More specifically, the base portion includes a pair of base rails, made of hardened steel, and the slider includes a pair of slider rails, also made of hardened steel, which are fitted into side grooves provided in the base portion and the slider respectively.
In the above structure, the base rails are made according to the following process. First, the base rails are formed by grinding or by a plasticity rolling process, and rolling body tracks are formed in the rails by a heat treatment and hardening process. Under this condition, the base rails are fitted into the base side grooves, and the rolling body tracks are ground to have a transverse cross-sectional shape of a Gothic style arch. The slider rails, which are formed by the same process, are fitted into the slider side grooves. To reduce adverse effects caused by a difference in the coefficients of linear thermal expansion of aluminum and steel, critical dimensional features of the actuator are established, such that the thickness D2 of the base rails (as well as the thickness of the slider rails) is set to be smaller than the diameter D3 of the balls, and equal to or less than 10% of a center distance L2 between the balls on respective sides of the slider, and further, the width B of the base rails is set to be equal to or less than twice the diameter D3 of the balls, thereby satisfying the relationships, D2<D3, D2≦0.1×L2, and B≦2×D3.
Noted advantages associated with the above structure are, (1) since most of the base and slider are made of aluminum and only the base rails and slider rails are made of steel, the overall structure is light in weight, (2) thermal expansion related problems are reduced because the portions associated with rolling of the balls are made of the same material on both the base and slider sides, (3) the mechanical strength of the steel base rail and the steel slider rail are high so as to ensure stable performance over time, and (4) owing to the critical dimensional features discussed above, it is possible to reduce the influence of dimensional changes caused by the difference in thermal expansion between aluminum and steel.
Nevertheless, in the actuator of U.S. Pat. No. 5,775,515, although attempts are made to minimize the adverse effects brought about by the difference in thermal expansion between aluminum and steel, a complete solution to this problem cannot be obtained. In particular, since the coefficients of linear thermal expansion differ markedly (23.6×10−6/° C. for aluminum alloys verses 10.7×10−6/° C. for steel), when moved into a hotter environment, the aluminum alloy portions of the base and slider, including the side grooves thereof in which the guide rails are positioned, expand roughly two times greater than the steel guide rails themselves, which leads to gaps occurring between the side grooves and the guide rails, along with potential dislodgement or loosening of the guide rails within the side grooves.
Moreover, a further problem results from the disclosed structure owing to the difference in Young's modulus, which is a measure of rigidity, between aluminum and steel. In particular, because the Young's modulus of a light aluminum alloy is only about one-third that of steel and other ferrous based metals, the cross-sectional shape and size of the base frame in U.S. Pat. No. 5,755,515 must be made much larger, and a complex hollowed structure must be used, in order to provide sufficient rigidity comparable to that of a steel frame, increasing both size and cost, as well as complexity in fabricating the actuator body.